Large quantities of hydrocarbons are trapped in geologic formations around the world. Crude oil and natural gas are the only hydrocarbons that naturally occur. These are viewed as strategic resources because of technological dependency on petroleum products for fuel and raw materials. Coal is a significant source of hydrocarbons for energy production and manufacturing.
Crude oil, natural gas, and coal are the most widely utilized sources of hydrocarbons because they are relatively inexpensive to find and refine. Thus, every spike in the price of common hydrocarbons stimulates interest in alternative sources of hydrocarbons.
Oil shale is such an alternative source. Oil shale is a hydrocarbon source rock composed of inorganic sedimentary particles and appreciable organic material. Some estimates suggest that global deposits of oil shale contain roughly three trillion barrels of recoverable hydrocarbons. Kerogen, the organic material in oil shale, is the solid precursor to crude oil, natural gas, and coal. Over geologic time, kerogen deep in the Earth decomposes under geothermal pressure and transforms into petroleum products.
This geologic process can be mimicked by retorting. Retorting is a method of extracting recoverable hydrocarbons that involves heating oil shale to several hundred degrees centigrade in the absence of oxygen. The kerogen in the oil shale decomposes into numerous hydrocarbon rich gases that are collected and liquefied. The liquefied “shale oil” is then refined and processed similarly to crude oil.
Unfortunately, many oil shale deposits contain as little as 25% kerogen. Further, there is a significant energy loss associated with heating the inert geologic materials in the oil shale to several hundred degrees centigrade. Thus, producing shale oil is not economically viable in regard to it's “energy return on energy invested” (EROEI), unless the price of crude oil is greater than about $75 a barrel. Retorting on a large scale is not desirable because of potential environmental pollution and heavy demand upon water resources. The environmental pollution is exacerbated because potentially valuable inorganic materials and metals are not recovered from the spent shale and advantageously disposed of.
The EROEI of oil shale can be improved by enriching the kerogen content prior to retorting. This enrichment is very problematic because kerogen is insoluble and impervious to organic solvents. One solution is to crush the oil shale in order to expose more kerogen during retorting. Other solutions include using acid treatments to reduce the amount of rock in a sample of shale.
Limited biotic decomposition of kerogen is possible (See, e.g., U.S. Pat. No. 2,641,565) but its use has apparently been economically unsuccessful, which is evidenced by its lack of pervasive commercial use over the last 100 years. There are known species of bacteria that are able to consume kerogen, but the bacteria excrete unknown compounds. On the other hand, while some bacteria have been genetically modified bacteria to excrete hydrocarbons, these do not consume kerogen.
Although producing hydrocarbons through biotic decomposition has not yet proven economically successful, there has been limited success in processing oil shale with microorganisms. For instance, the ability of certain bacteria to excrete acid has been applied to dissolve the inorganic sedimentary matrix in oil shale (See, U.S. Pat. No. 3,982,995). That inventive method produces sponge-like shale with increased surface area to promote combustion of the kerogen.
Coal, which is a solid derivative of kerogen, can be processed similarly to oil shale in order to obtain liquid and gaseous hydrocarbons. All methods of coal liquefaction and gasification require significant heating of the coal. This leads to the same problems experienced as with shale oil including low EROEI and extensive environmental pollution.
There are two methods of coal liquefaction, direct liquefaction (DCL) and indirect liquefaction (ICL). The DCL process is operated between 302-572° F. (150-300° C.) and at a pressure range between one and several tens of atmospheres. Higher pressures would be favorable, but the benefits may not justify the additional cost of high-pressure equipment. The DCL process, like retorting used in oil shale, first coverts the coal to a gas, then condenses it to a liquid. The ICL process avoids the gasification process by using solvents or catalysts in a high pressure and high temperature process.
Cellulose (C6H10O5) does not contain hydrocarbons but is instead a carbohydrate. Hydrocarbons and carbohydrates are not interchangeable and cannot be conflated. However, microorganisms such as yeast can convert cellulose and water into liquid hydrocarbons through fermentation. Agricultural waste and recycled wood are common sources of cellulose for such purposes. As with methods of hydrocarbon recovery, manufacture of hydrocarbons from cellulose has significant drawbacks including low EROEI and extensive environmental pollution.
Conversion of a cellulose biomass into biofuel is currently achieved by thermal conversion. The major methods of thermal conversion are combustion in excess air, gasification in reduced air and pyrolysis in the absence of air. A number of combustion technologies are available, all requiring boilers or fluid bed combustors. The latter can produce fuels with lower NOx levels. Co-Firing, using a fossil fuel is also used as a combustion method. Gasification produces a lower calorific value but can still be used as a fuel for boilers, engines, and possibly combustion turbines but requires cleaning the gas stream of tars and particulates. Pyrolysis is thermal degradation in the absence of air. It produces a solid char, gas and a liquid bio-oil. The bio-oil can act as a liquid fuel.
A common thread among the efforts to obtain hydrocarbon fuels from oil shale, coal, tar sands, and cellulose has been the excessive cost. We know the cost to produce a barrel of kerogen using ex situ retorting methods is between $156 and $200 a barrel. With crude oil selling on the world markets between $40 and $75 a barrel, the cost to produce a barrel of kerogen hydrocarbon has a negative EROEI. No matter how efficient current in situ or ex situ retorting methods are, producing a barrel of hydrocarbon(s) using known methods is cost prohibitive. Each of oil shale, coal, tar sands and cellulose requires heating the feedstock to hundreds of degrees. In addition, the condensation of hydrocarbon gases from oil shale, coal and tar sands to a liquid form is an expensive process. What is needed is a method to produce hydrocarbon fuels from these various feedstocks with a lower EROEI than gasoline.